Possibly dumb question: can I use two cable runs for each channel, not bi-wired?

Yes. Until I found the best single cable for my system, I had it bi-wired using two different makes to achieve the best sound. I just made sure to use bananas for one run and spades for the other. Or, you can use bare wire and spades, or bare wire and bananas, but I wouldn't try both with spades. I don't think you'd get a good enough grip that way.

In the end, it tended to be a bit "phasey", for lack of a better word, but it sounded nice.

All the best,
Nonoise

sure you can, but the question I have is why?

stuartk
If you double the wire as you have suggested then the result electrically is that is you halve the impedance of the cable. This may tighten the bass response since the impedance of the cable is halved.
Try it, and use your own judgement.

Good comments by the others. I would add that the only situation in which I would definitely avoid doing that is if the cable happens to be one of the few cable types that have ultra-high capacitance, such as Goertz, or the old Polk Cobra cable. Paralleling two identical runs, as you are proposing, will double the cable capacitance seen by the amplifier, and if the capacitance of the cable is very high to begin with that could result in adverse effects up to and including oscillations and amplifier damage.

With the great majority of speaker cables, however, the doubled capacitance will not be of any significance, and the halving of resistance and inductance that will result will (at least slightly) work in the direction of improving the accuracy of transmission of the signal, as well as resulting in a slight increase in woofer damping. Of course, those differences may not always be subjectively preferable, so as suggested above it may be a good idea to try it both ways.

Regards,
-- Al

Thanks for your comments. 

1) Al: the maker of this particular cable combines two runs of it into a shotgun for their more expensive product, so I'm assuming it's OK. 

2) the reason I asked this question, is that I've gotten conflicting opinions re: bi-wiring: one says I have to use tow different gauges of wire for woofers and tweeters, while another says I can use the same gauge. If I can use the existing cable to bi-wire, I will do that. I'm just not clera about what I can or can't do in this regard.